1. Field of the invention
This invention relates to a combustion system for gas turbine gas generators, gas turbine engines, or other heat devices, which can produce significant advantages including low levels of pollutants, namely oxides of nitrogen, carbon monoxide, and unburned hydrocarbons. In one aspect, the present invention relates to a system, process, and apparatus for combusting fuel in a gas turbine or gas generator module which significantly lowers pollutants by providing a nearly constant fuel/air ratio in the combustion zone at all engine operating conditions in addition to thoroughly premixing the fuel and air prior to combustion and, when necessary, completely vaporizing a liquid fuel. In another aspect, the present invention relates to single stage, controllable fuel/air ratio combustors for gas turbine fuel/air premixer assemblies. In another aspect, the present invention relates to a mixing tube in a premixer chamber for a combustor system having a flow axis, an inlet port adjacent one axial mixing tube end, and a nozzle assembly at the opposite axial mixing tube end, the mixing tube inlet being flow connected to a source of compressed air and a source of fuel, the mixing tube being connected to the liner housing at a housing inlet port, and the nozzle assembly extending along the mixing tube flow axis into a combustion chamber and having at least one port for distributing the fuel/air mixture to a combustion zone.
2. Description of the Art
Although gas turbine devices such as engines and gas generators do not produce the majority of the nitrogen oxide emissions released into the earth's atmosphere, reducing those emissions will reduce the total and, in that regard, many countries have enacted laws limiting the amounts that may be released. The reaction of nitrogen and oxygen in the air to form nitrogen oxides, like almost all chemical reactions, proceeds faster at higher temperatures. One way to limit the amount of NO, formed is to limit the temperature of the reaction. The NO.sub.x produced in gas turbine devices is produced in the combustion process where the highest temperature in the cycle normally exists. Therefore, one way to limit the amount of NO.sub.x produced is to limit the combustion temperature.
Various attempts have been made to limit the combustion temperature and thereby NO.sub.x production in both "single stage" combustors (i.e., those having only a single combustion zone where fuel and air are introduced) and "multistage" combustors, including pilot burners where several, serial connected combustion zones having separate fuel and air introduction means are used. U.S. Pat. Nos. 4,994,149, 4,297,842, and 4,255,927 disclose single stage gas turbine combustors where the flow of compressed air to the combustion zone and the dilution zone of an annular combustor are controlled to decrease the concentration of NO.sub.x in the turbine exhaust gases. In the above combustors, essentially unmixed fuel and air are separately admitted to the combustor, with mixing and combustion consequently occurring within the same chamber. See also Japanese Laid-Open No. 55-45739. U.S. Pat. Nos. 5,069,029, 4,898,001, 4,829,764, and 4,766,721 disclose two stage combustors. See also German Gebrauchsmuster, 99215856.0. Again, however, fuel and air are provided to each stage at least partially unmixed with complete mixing occurring within the respective combustion zones.
Attempts also have been made to utilize separate premixer chambers to provide a premixed fuel-air flow to a combustor. Japan Laid-Open Application No. 57-41524 a discloses a combustor system which appears to premix only a portion of the total fuel flow to a multistage can-type combustor in a separate mixing chamber prior to introduction to the staged combustion chambers. In U.S. Pat. No. 5,016,443, a large number of separate fuel nozzles are used to inject fuel into an annular premixer chamber. However, the complexity of the above constructions employing multiple fuel nozzles and fuel splitting devices can lead to control difficulties, as well as a high initial cost.
Accordingly, it is desired to design a combustor apparatus for use with gas turbine gas generator and engine modules, which apparatus can result in low emissions of pollutants and fuel by-products over the entire operating range of the gas turbine gas generator or engine module.
It is further desired to advantageously design a premixer system that can reduce "flash backs" from the combustor into the premixer, which can occur when the flame speed is greater than the velocity of the fuel/air mixture in the premixer. Flash backs can adversely affect the mechanical integrity and performance of the premixer system and related structure. It is further desired to advantageously design a premixer system that can reduce flow separation in the premixer resulting from the geometrical configuration of the premixer components. Flow separation can cause flash backs into the premixer. It is further desired to design a premixer system that can reduce pulsations in the delivery of fuel/air mixture from the premixer into the combustion chamber, which can occur due to the minor variations in the compressed air velocity in the premixer. Pulsations can adversely affect the combustor liner and engine structure. Each of the above described conditions, if they occur, can adversely affect the mechanical integrity and performance of the gas turbine. It is further desired to advantageously design a premixer system that can deliver fuel/air mixture into the combustion chamber in a manner that reduces the impingement of flow onto the combustor liner while maintaining a comparatively simple geometric configuration of the overall design. Impingement of the flow onto the liner wall can lead to carbon build up and decrease heat transfer performance and increase thermal fatigue.
It is further desired to design an apparatus that is relatively less complex than other state of the art annular combustor apparatus and systems thereby facilitating ease of operation, lower initial cost and maintenance of the apparatus, and substantially improved fuel/air control by the avoidance of matching a large number of separate premixers.